Due to increased demand of bandwidth and bit rates per channel, access technologies are moving forward to coherent systems. This requires the detection of complex phase modulation with wide spectral efficiency. Furthermore, the networks' efficiency is, in turn, being enhanced by reducing their channel spacing (ultra-Dense-WDM-PON, WDM-PON being short for Wavelength Divisional Multiplexing Passive Optical Network). Therefore, it has become usual to work with a distance between channels of 50 GHz and it tends to be rapidly reduced to 12.5 GHz and beyond.
The spectra measurement of these optical networks is done by Optical Spectrum Analysers, OSA. Depending on the technology that they use, all existing OSAs can be classified in four categories.                Diffraction grating-based OSA: It is the most common one. It uses a monochromator as the tunable optical filter.        Interferometer-based OSA: It may use either Fabry-Perot or Michelson interferometry.        Brillouin-based OSA: It works with the Stimulated Brillouin Scattering non-linear effect.        Coherent detection-based OSA: It combines a tunable narrow-linewidth laser (local oscillator) with the input signal.        
These are all associated with various drawbacks. Diffraction grating-based OSA cannot be used for coherent systems, since it does not detect data phase information. It has a limited resolution due to grating's physical dimension; limited accuracy since expensive mechanical elements are needed for a high accuracy; and limited sweep time (in the order of seconds) due to mechanical movement. Interferometer-based OSA may achieve high resolution, but it has low dynamic range. It presents higher accuracy than the diffraction grating-based OSA but it is still limited by mechanical dimensions. Brillouin-based OSA may be used as coherent detector, but it needs a high power tunable laser. Moreover, these lasers are normally extremely expensive. This technology requires complex signal processing due to the non-linear effect. Coherent detection-based OSA is expensive and presents limited sweep time due to the mechanical components of the External Cavity Laser, ECL, tunable laser.